Human ears are sensitive to artifacts in sound. These artifacts include but are not limited to different types of noises, sampling/digitization/compression artifacts, and echoes. Each of these artifacts originates from different source, may have different correlation with the intended sound, and may thus be filtered or suppressed based on different corresponding technological approaches. For example, noise may be completely random and may be of different “colors” as determined spectrally, e.g., noises may be white, pink, brown, blue, violet, and grey, all with respect to the shape of frequency spectra of the noises. Echoes, particularly acoustic echoes, are sound waves that are correlated with existing intended sound. Because random noises and correlated echoed are of distinct natures, suppression techniques that are applicable to random noise may not be effective against echoes.
Echoes may occur in various forms in different application environments. For example, in a telephone/conference system, echoes may be produced in the far end of a telephone call by transmitting, by the near end terminal, acoustic signal originated from the far end but picked up by a microphone in the near end, to the far-end together with the near end acoustic signal. In a multimedia system having one or more speakers in a large room, such as a conference hall or a movie theater (herein generically referred to as “room”), noticeable echoes may be produced simply by walls or other structures in the room that reflect sound. A typical consumer of multimedia in the modern age often expects artifact-free experience in sound, necessitating a desire to provide acoustic echo reduction for entertainment systems. While special sound absorbing materials may be used for constructing, e.g., a conference room, a movie theater, or a large residential multimedia room, echo suppression may not be adequate based on these mechanical approaches or the construction may be economically infeasible. Thus, it is desirable to reduce echoes in these situations using electronic means, preferably designed into the acoustic and electronic audio devices already equipped in the room.